The present invention generally relates to memory devices and more particularly to a ferroelectric memory device that stores information in the form of polarization of a dielectric film.
Semiconductor memories are used extensively in human society, together with central processing units (CPUs), to form various electronic apparatuses and systems that range from main frame computers to home electronic products. In DRAMs in particular, there is a persisting trend to increase the memory capacity, and this trend is now accelerating. Today, DRAMs having a memory capacity of 256 Mbits or 1 Gbits are the target of intensive research.
On the other hand, conventional DRAMs have a complex structure, and because of this, it is expected that such an increase of the memory capacity reaches a limit in one or two generations. In DRAMs, information is stored in a minute memory cell capacitor in the form of electric charges, while miniaturization of the memory device inevitably invites a decrease of the capacitance and hence the electric charges stored in the memory cell capacitor. Although there are attempts to defer the decrease of capacitance of memory cell capacitors as much as possible by devising a complex capacitor structure, such an approach is reaching a limit. In order to realize a DRAM having a memory capacity of 256 Mbits or more, it is necessary to construct the semiconductor memory device based upon a new principle.
On the other hand, there are non-volatile semiconductor memories such as EPROM or flash memory that are capable of retaining information even when the electric power is turned off. As such non-volatile semiconductor memories are compact and solid and consume little electric power, they are expected to replace conventional magnetic memories such as hard disk drives or floppy disks. On the other hand, such conventional non-volatile semiconductor memories store information in a floating gate electrode in the form of electric charges, and thus, such conventional non-volatile semiconductor memory devices have a drawback in that they require precise control of electric charge injection and removal in order to avoid the problem of excessive erasing of information. Further, the number of times the data is rewritten by way of charge injection is limited to 10.sup.5 -10.sup.6 times.
Meanwhile, it is known that ferroelectric materials have a large capacitance that is several ten to several hundred times as large as the capacitance of the dielectric material such as SiO.sub.2 or SiN that are used conventionally in DRAMs. Further, such ferroelectric materials show a spontaneous polarization, and thus they can be used for storing information in the form of polarization. As spontaneous polarization of the ferroelectric materials does not disappear even when the electric field is removed, it is possible to construct a non-volatile semiconductor memory device by using a ferroelectric material.
As the semiconductor memory device that uses a ferroelectric film, a device structure called MFS (metal ferroelectric semiconductor) transistor is proposed, wherein the MFS transistor is a transistor having a structure identical to that of a MOS FET, except that the gate oxide film thereof is replaced by a ferroelectric film. In this transistor, information is stored in the foregoing ferroelectric film in the form of polarization. For example, a structure is proposed in which a single crystal TGS (triglycine sulfate) is used for the ferroelectric gate. Further, there is a report that a successful operation is obtained in a device in which the gate oxide film is replaced by a ferroelectric film in a Si MOS transistor.
In the known structure of the ferroelectric semiconductor memory device in which the gate oxide film is simply replaced by a ferroelectric film, it is necessary to deposit the ferroelectric film directly upon a semiconductor layer such as Si or GaAs. When a ferroelectric film, typically an oxide such as PZT or BaTiO.sub.3, is deposited directly on a semiconductor layer of non-oxide, there occurs a problem of diffusion of oxygen atoms from the dielectric film into the semiconductor layer. Thereby, there is a substantial risk that the electric charges are trapped at the semiconductor-ferroelectric interface by defects that are created by the diffused oxygen atoms. When such a trapping of the electric charges occurs, the operation of the memory device is inevitably deteriorated and becomes unstable. Further, the deposition of high quality dielectric films requires a deposition temperature higher than 500.degree. C., while such a high temperature treatment tends to invite interdiffusion of other elements between the dielectric film and the semiconductor layer.
In order to realize a reliable ferroelectric semiconductor memory device, it has been necessary to carefully select the ferroelectric material such that no substantial deterioration occurs in the semiconductor layer and also in the ferroelectric film.
Further, such conventional ferroelectric semiconductor memories have another serious drawback in that the voltage applied to the gate electrode for causing a polarization in the ferroelectric film, is more or less applied directly to the semiconductor substrate underneath the ferroelectric film, more specifically the surface part of the substrate where oxygen contaminants are contained, without inducing any substantial electric field in the ferroelectric film. It should be noted that the ferroelectric film has a large dielectric constant while the semiconductor layer has a much smaller dielectric constant. In other words, there occurs no substantial voltage drop across the ferroelectric film even when a write control voltage is applied to the gate electrode. Thus, the conventional ferroelectric semiconductor memory device has suffered from a problem that it requires a large write control voltage.